1. Field of Invention
Inventive aspects are associated with shape sensing, more particularly to sensing position and orientation at various points of a kinematic chain, and still more particularly to sensing the shape of a flexible body and sensing the position and orientation of an end effector component of a surgical instrument or of an entry guide for one or more surgical instruments in a telerobotic surgical system.
2. Background Art
FIG. 1 is a diagrammatic view of an optical fiber core portion 100. The surrounding cladding and fiber are omitted for clarity. Two fiber Bragg gratings (FBG's) 102a,102b are shown formed in fiber core portion 100, which are illustrative of many such FBG's typically formed along the full length of a core. The many vertical lines shown in each FBG 102 represent the changes in refractive index that characterize an FBG. As shown in FIG. 1, the FBG's 102a,102b are separated by a tether segment 104, which is completely transmissive.
As is known, each of the FBG's 102 may be interrogated for strain information. In a fiber that contains two or more cores, with FBG's in each core positioned at the same location along the fiber, the fiber's bend direction and amount of axial twist may be determined from the strains in each core's FBG's. From the strain information from each core at each FBG location, and from the known tether segment length (e.g., 5 mm, 1 cm), the position of the location with the next set of FBG's can be estimated. In this way, the fiber shape associated with the interrogated FBG's may be determined. U.S. Pat. App. Publ. No. US 2006/0013523 A1 (filed 13 Jul. 2005), which is incorporated herein by reference, discloses a fiber optic position shape sensing device and method. In one instance, strain information is used to determine the bend angle between two links, as described in U.S. Pat. App. Publ. No. US 2007/0156019 A1 (filed 20 Jul. 2006), which is incorporated herein by reference. Such bend information may be used in forward kinematic calculations to determine the position of the distal end of a kinematic chain (a set of links coupled by one or more movable mechanical constraints). One non-limiting example of such a kinematic chain is a minimally invasive surgical instrument with one or more revolute joints.
In one instance, a curvilinear coordinate system is defined with an origin at the connection where the fiber is joined to the interrogator unit (i.e., at the proximal end of the fiber). In addition, a Cartesian frame is also defined as a base reference frame having an origin coincident with the curvilinear coordinate system's origin. Using known techniques, each of the FBG's 102 are interrogated for strain information.
Known techniques to interrogate the FBG's by processing the light reflected back when an optical light source is coupled with the fiber are described in, for instance, Andreas Othonos & Kyriacos Kalli, Fundamentals and Applications in Telecommunications and Sensing Ch. 7, 301-388 (Arthech House 1999), which is incorporated herein by reference. Such interrogation techniques include on the use of edge filters, tunable filters, interferometers, tunable lasers, and CCD spectrometers. Each technique provides different spatial resolution in the measurement of strain along a fiber core, different speeds of interrogation, i.e., update of the measurement, and different levels of immunity to disturbances to the strain measurements such as those produced by variation of temperature, light polarization, source light variations, and losses of light along the fiber. Among the interferometric techniques that are based on detection of the phase of the reflected light are Optical Time Domain Reflectometry and Optical Frequency Domain Reflectometry (OFDR), as described in U.S. Pat. No. 5,798,521 (filed 27 Feb. 1997) and in U.S. Pat. App. Publ. No. US 2007/0065077 A1 (filed 26 Sep. 2006), which are incorporated herein by reference.
To determine the fiber's approximate shape, the strain information measured at each FBG location is used to determine the approximate local bend for the length of fiber without FBG's (e.g., over a 1 cm tether segment). For example, the strain information from three cores in a fiber is used to compute the plane and the bend radius of the fiber. Segments are defined at various locations along the fiber, and each segment ends at a co-located ring of FBG's in the three cores. Given the Cartesian x,y,z position of the FBG ring being processed (i.e., the segment end position), the position of the next FBG ring can be computed with simple geometry. The position of the first segment's end location with respect to the base frame is then determined from the first segment's bend information. Next, strain information for the second segment is processed to determine the second segment's bend. The second segment's bend information is combined with the position of the first segment's end location to determine the second segment's end location position with respect to the base frame. Thus the position of each segment end location is determined with respect to the base frame, and the position information is used to determine the approximate shape of the fiber.
There are, however, disadvantages to current optical fiber shape sensing methods. To begin with, such methods are based on the average strain measured in each FBG, and so the FBG size limits the measurement resolution. In addition, the shape (or state) of the fiber is reconstructed as a vector of equally spaced three-dimensional (3D) points in world coordinates. The result is a large data set that gets larger as measurement resolution is increased. Also, the resolution of the 3D points is limited by the spacing between FBG's. Further, in these methods there is an assumption that the length of fiber between measured FBG's has a constant bend radius, and this assumption can reduce the accuracy of the sensed shape as compared with the fiber's actual shape. Another disadvantage is that the computation of the fiber tangent vector at a particular fiber location (i.e., the direction the fiber is pointing at that location) requires differentiation of the 3D points. This differentiation delivers even lower measurement resolution. Yet another disadvantage is that the 3D point data set is not in a form that is required for the type of processing needed when the fiber is embedded into a kinematic chain, and therefore the kinematic chain's pose has to be inferred from the sensed fiber position. This limitation is especially true if the fiber is embedded in a kinematic chain but is allowed to slide with reference to one or more links. In such configurations, the fiber may not follow a path that exactly corresponds to a bend in one or more of the kinematic chain's joints, and friction between the fiber's surface and a surrounding conduit through one or more links may influence the fiber's shape.
What is needed, therefore, is a more effective and accurate way to determine the shape of an optical fiber and a more effective way of producing the shape information for use in determining the position and orientation of all the links of a kinematic chain, i.e., the pose of a kinematic chain, that is associated with the fiber. These needs are especially true for various real time implementations, such as for telerobotically controlled minimally invasive surgical instruments.